1. Field of the Invention
The present invention relates generally to an implantable delivery device, and more particularly to an exit port, such as a slit orifice for an implantable osmotic delivery device which has a variable size.
2. Description of the Related Art
Controlled delivery of beneficial agents such as drugs in the medical and veterinary fields has been accomplished by a variety of methods. One approach for delivering a beneficial agent involves the use of implantable diffusional systems. For example, subdermal implants for contraception are described by Philip D. Darney in Current Opinion in Obstetrics and Gynecology, 1991, 3:470-476. Norplant.RTM. requires the placement of 6 levonorgestrel-filled silastic capsules under the skin. Protection from conception for up to 5 years is achieved. The implants operate by simple diffusion, that is, the active agent diffuses through the polymeric material at a rate that is controlled by the characteristics of the active agent formulation and the polymeric material.
Another method for controlled prolonged delivery of a beneficial agent involves the use of an implantable osmotic delivery system. Osmotic delivery systems are very reliable in delivering the beneficial agent over an extended period of time. The osmotic pressure generated by an osmotic pump also produces a delivery rate of the beneficial agent into the body which is relatively constant as compared with other types of delivery systems.
In general, osmotic delivery systems operate by imbibing fluid from the outside environment and releasing corresponding amounts of the beneficial agent. Osmotic delivery systems, commonly referred to as "osmotic pumps", generally include some type of a capsule having walls which selectively pass water into an interior of the capsule which contains a water-attracting agent. The absorption of water by the water-attracting agent within the capsule reservoir creates an osmotic pressure within the capsule which causes the beneficial agent to be delivered from the capsule. The water-attracting agent may be the beneficial agent delivered to the patient, however, in most cases, a separate agent is used specifically for its ability to draw water into the capsule.
When a separate osmotic agent is used, the osmotic agent may be separated from the beneficial agent within the capsule by a movable dividing member or piston. The structure of the capsule is such that the capsule does not expand when the osmotic agent takes in water. As the osmotic agent expands, it causes the movable dividing member or piston to move, which in turn causes the beneficial agent to be discharged through an orifice at the same volumetric rate that water enters the osmotic agent by osmosis.
The orifice controls the interaction of the beneficial agent with the external fluid environment. The orifice serves the important function of isolating the beneficial agent from the external fluid environment, since any contamination of the beneficial agent by external fluids may adversely affect the utility of the beneficial agent. For example, the inward flux of materials of the external fluid environment due to diffusion or osmosis may contaminate the interior of the capsule, destabilizing, diluting, or otherwise altering the beneficial agent formulation. Another important function of the orifice is to control or limit diffusional flow of the beneficial agent through the orifice into the external fluid environment.
In known delivery devices, these functions have typically been performed by flow moderators. A flow moderator may consist of a tubular passage having a particular cross sectional area and length. The cross sectional area and length of the flow moderator is chosen such that the average linear velocity of the exiting beneficial agent is higher than that of the linear inward flux of materials in the external environment due to diffusion or osmosis, thereby attenuating or moderating back diffusion and its deleterious effects of contaminating the interior of the osmotic pump.
In addition, the dimensions of the flow moderator may be chosen such that the diffusive flux of the beneficial agent out of the orifice is small in comparison to the convective flux. FIG. 1 is a graph showing the relationship between the orifice dimensions and drug diffusion as a percentage of pumped or connective delivery for one set of pumping rates and drug diffusivity. FIG. 1 shows, for example, that the diffusive flux of the beneficial agent can be kept to less than 10% of the convective flow using an orifice having a diameter of 5 mils and a length of at least 0.6 cm, or an orifice having a diameter of 10 mils and a length of at least 2.4 cm.
One problem with flow moderators, however, is that the passage may become clogged or obstructed with particles suspended in the beneficial agent or in fluid from the external environment. Such clogging may be reduced or eliminated by increasing the diameter of the passage to 5 mil or more, for example. However, as shown in FIG. 1, this increase results in a greater rate of diffusion of the beneficial agent out of the osmotic pump. A corresponding increase also occurs in the back diffusion of the external fluid into the osmotic pump which may contaminate the beneficial agent and adversely affect the desired delivery rate of the beneficial agent. Tolerances during fabrication also frequently dictate that the orifice diameter be greater than about 5 mils.
Systems with a long straight flow moderator are also impractical for implantation applications because they increase the size of the implant significantly making the system difficult to implant.
Current flow modulators also cause separation of beneficial agents which contain suspensions of bioactive macromolecules (proteins, genes, etc.). When such suspensions pass along a restriction in current flow modulators, the suspension separates and the delivery concentration of bioactive macromolecules varies.